Tech Briefs

A Lightweight Ambulatory Physiological Monitoring System

Readings of multiple sensors are recorded for subsequent playback and analysis.

The Autogenic-Feedback System-2 (AFS-2) is a biomedical instrumentation package that was designed and built at Ames Research Center for use during the September 1992 Spacelab-J (STS-47) mission. The AFS-2 performed successfully during that mission and was rated by members of NASA's Astronaut Office as the best instrument of its kind because of its high data quality, ease of operation, and minimal time for setup and operation. Because of its small size, this system offers comfort and mobility greater than those of other systems developed for the same purpose.

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The AFS-2 is a self-contained, battery-powered, ambulatory, physiological monitoring system. It can continuously monitor, display, and record up to nine channels of physiological data for up to twelve hours on a single change of batteries. Sensors and transducers, placed in various locations on the subject (e.g., an astronaut), monitor the physiological signals. A wrist display unit displays the subject's physiological parameters in numerical form. The AFS-2 records all acquired information on a data instrumentation tape by use of a modified nine-channel frequency-modulation data recorder. Researchers can thereafter play back the tape to extract the data and analyze the subject's performance.

By use of various sensors and transducers (see figure), the AFS-2 monitors the following parameters:

  • Blood Volume Pulse (BVP) — A miniature infrared emitter-detector pair mounted in a ring that is worn on the small finger of the left hand detects changes in blood-vessel volume in the hand.
  • Skin Temperature (TEMP) — A miniature sensor, mounted in the same ring used for measuring BVP, measures the temperature of the skin.
  • Skin Conductance Level (SCL) — A pair of electrodes mounted on the left wrist monitors changes in the electrical conductivity of the skin.
  • Respiration (RESP) — A thin piezoelectric film sandwiched between two flexible rubber housings and strapped across the diaphragm measures both the range and frequency of respiratory cycles.
  • Electrocardiography (ECG) — Three standard electrodes placed at the AvR, AvL, and AvF chest nodes monitor the electrical impulses of the heart.
  • Acceleration (ACCEL) — An accelerometer attached to a flexible cotton headband measures the motion of the subject's head along three axes.

The AFS-2 operates in either of two display modes: one for treatment subjects and one for control subjects. In the treatment mode, the wrist display unit displays all indications of system status, malfunctions, and monitored physiological data. In the control mode, only system-status and malfunction indications are visible to the subject.

The data recorder receives the BVP waveform, skin temperature, skin conductance, respiration waveform, ECG waveform, acceleration signals, time and date, specified events, and session timing data and records them on a standard-sized magnetic data instrumentation cassette tape. The cassette can be played through a separate playback unit for subsequent analysis at speeds up to 32 times the speed at which the data were recorded.

The AFS-2 is divided into three general subsystems: the wrist display unit, the belt assembly, and the garment and cable harness assembly. The wrist display unit, fastened to the left sleeve of the AFS-2 garment, includes a small liquid-crystal display device that presents physiological and system-status information to the user. The belt assembly is worn around the waist and over any clothing. The belt assembly comprises belt electronic circuitry (signal-conditioning amplifiers, an analog-to-digital converter, and a microcontroller), a battery pack, the data recorder, and an interface cable. A modular design distributes the weight of the belt assembly evenly around the waist. The components of the system are interconnected through easily mated and demated connectors; this design feature minimizes the time needed for donning the AFS-2. The battery pack supplies power to all subsystems, including the data recorder. The battery pack features a clip-on design for fast and easy replacement of exhausted batteries. The AFS-2 garment assembly is worn on the upper body and covers the torso and left arm. The garment assembly comprises the garment, the cable harness, the respiration transducer, the accelerometer, and the ring transducer.

This work was done by Patricia Cowings, Scott Jensen, Dave Bergner, and William Toscano of Ames Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Test and Measurement category.

This invention has been patented by NASA (U.S. Patent No. 5,694,939). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Ames Research Center, (650) 604-5104. Refer to ARC-14048-1.

This Brief includes a Technical Support Package (TSP).

A Lightweight Ambulatory Physiological Monitoring System (reference ARC-14048-1) is currently available for download from the TSP library.

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